As background, the advantages of a environmental scanning electron microscope over the standard scanning electron microscope ("SEM") lie in its ability to produce high-resolution electron images of moist or nonconductive specimens (e.g., biological materials, plastics, ceramics, fibers) which are extremely difficult to image in the usual vacuum environment of the SEM. The environmental scanning electron microscope allows the specimen to be maintained in its "natural" state, without subjecting it to the distortions caused by drying, freezing, or vacuum coating normally required for high-vacuum electron beam observation. Also, the relatively high gas pressure easily tolerated in the ESEM specimen chamber acts effectively to dissipate the surface charge that would normally build up on a nonconductive specimen, blocking high quality image acquisition. The ESEM also permits direct, real-time observation of liquid transport, chemical reaction, solution, hydration, crystallization, and other processes occurring at relatively high vapor pressures, far above those that can be permitted in the normal SEM specimen chamber.
Typically, in an ESEM, the electron beam is emitted by an electron gun and passes through an electron optical column of an objective lens assembly having a final pressure limiting aperture at its lower end thereof. In the electron optical column, the electron beam passes through magnetic lenses which are used to focus the beam and direct the electron beam through the final pressure limiting aperture.
The beam is subsequently directed into a specimen chamber through the final pressure limiting aperture wherein it impinges upon a specimen supported upon a specimen stage. The specimen stage is positioned for supporting the specimen approximately 1 to 10 mm below the final pressure limiting aperture so as to allow the beam of electrons to interact with the specimen. The specimen chamber is disposed below the optical vacuum column and is capable of maintaining the sample enveloped in gas, preferably nitrogen or water vapor, at a pressure of approximately between 10.sup.-2 and 50 Torr in registration with the final pressure limiting aperture such that a surface of the specimen may be exposed to the charged particle beam emitted from the electron gun and directed through the final pressure limiting aperture.
The typical specimen stage previously used in an environmental scanning electron microscope to image samples at high temperatures is illustrated in FIG. 1. In that prior specimen stage, the electron beam 1 strikes a surface of the specimen 2 which is supported in a recess 3 of a sample cup 4 which is supported in a depression 5 of an insulating jacket 6. An annular heating assembly 7 is provided in the depression 5 between the insulating jacket 6 and the sample cup 4, which is best shown in FIG. 2. This heater assembly 7 is formed of an annular toroidal ceramic former 8 having a heater wire 9 wound therearound such that the heater wire 9 is provided remote from the sample 2 and does not extend above the sample. In the prior specimen stage of FIG. 1, a heat shield 9a is positioned on top of the sample cup 4, the insulated jacket 6, and the heater assembly 7 which has a central opening 9b such that the electron beam can pass therethrough and strike the specimen 2.
For the reasons set forth above and below, the prior specimen stage for an environmental scanning electron microscope shown in FIGS. 1 and 2 has disadvantageously been unable to achieve sample temperatures of at least 1500.degree. C. First, the heater wire 9 cannot be made of tungsten because it will oxidize in the water vapor of the environmental scanning electron microscope. Hence, platinum wire must be used, but platinum melts at 1700.degree. C. Second, the heater is provided too remote from the sample and is significantly hotter than the sample. Finally, heat is radiated and convected from the surface of the sample as is represented by arrow a in FIG. 1 so that the bottom of the sample is hotter than the top, but the user is observing the top of the sample and the top surface cannot then be maintained at temperatures of at least 1500.degree. C. due to this radiant heat loss. Hence, utilizing the toroidal heater of FIGS. 1 and 2 in an environmental scanning electron microscope failed to achieve a sample temperature of 1500.degree. C.